U.S. patent number 3,687,219 [Application Number 04/831,402] was granted by the patent office on 1972-08-29 for ultrasonic beam expander.
This patent grant is currently assigned to Holotron Corporation. Invention is credited to Gary N. Langlois.
United States Patent |
3,687,219 |
Langlois |
August 29, 1972 |
ULTRASONIC BEAM EXPANDER
Abstract
A single compact unit for generating and controlling an
ultrasonic beam which includes an ultrasonic crystal generator and
a solid ultrasonic lens with a liquid ultrasonic energy coupling
therebetween which is contained in a housing joining the crystal
and the lens. An antireflection coating on the ultrasonic lens
surfaces improves transmission efficiency.
Inventors: |
Langlois; Gary N. (Richland,
WA) |
Assignee: |
Holotron Corporation
(Wilmington, DE)
|
Family
ID: |
25258969 |
Appl.
No.: |
04/831,402 |
Filed: |
June 9, 1969 |
Current U.S.
Class: |
181/176; 367/150;
73/644; 367/152 |
Current CPC
Class: |
G10K
11/30 (20130101) |
Current International
Class: |
G10K
11/30 (20060101); G10K 11/00 (20060101); G10k
011/00 (); G10k 011/06 () |
Field of
Search: |
;340/8LF,8MM,8L,10
;181/.5,.5A |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hubler; Malcolm F.
Claims
What is claimed is:
1. An ultrasonic lens, comprising,
a solid metal member including aluminum having surfaces shaped to
provide desired refraction of ultrasonic wave energy, and
a thin coating of graphite deposited upon said surfaces.
2. An ultrasonic lens according to claim 1 wherein said thin
coating of graphite has a thickness substantially equal to an odd
multiple of quarter wavelengths of ultrasound for which said lens
is designed to be used.
3. Apparatus for generating a controlled beam of ultrasonic energy,
comprising,
a solid ultrasonic lens having a convex incident surface and a
concave exit surface on opposite sides thereof, said concave exit
lens surface being shaped to form a substantially collimated output
beam when said exit surface contacts water,
means for generating an ultrasonic energy beam including a flat
transducer which generates a collimated beam,
a housing joining said generating means and said lens in a fixed
spatial relationship in a manner to direct said ultrasonic energy
beam to strike said lens incident surface, and
a coupling liquid sealed in said housing, whereby said liquid
transmits said ultrasonic energy from said transducer to said
incident ultrasonic lens surface.
4. Apparatus for generating a controlled beam of ultrasonic energy,
comprising,
a solid ultrasonic lens having a convex incident surface and a
concave exit surface on opposite sides thereof, said solid
ultrasonic lens being constructed of a material which is
characterized by having a speed of sound therein at least twice
that of water,
means for generating an ultrasonic energy beam,
a housing joining said generating means and said lens in a fixed
spatial relationship in a manner to direct said ultrasonic energy
beam to strike said lens incident surface, and
a coupling liquid sealed in said housing, said coupling liquid
being characterized by having a speed of sound therein which is
significantly less than that of water, whereby said liquid
transmits said ultrasonic energy from said transducer to said
incident ultrasonic lens surface.
5. Apparatus for generating a controlled beam of ultrasonic energy,
comprising,
a solid ultrasonic lens constructed of a material including
aluminum and having a convex incident surface and a concave exit
surface on opposite sides thereof,
means for generating an ultrasonic energy beam,
a housing joining said generating means and said lens in a fixed
spatial relationship in a manner to direct said ultrasonic energy
beam to strike said lens incident surface, and
a coupling liquid sealed in said housing, whereby said liquid
transmits said ultrasonic energy from said transducer to said
incident ultrasonic lens surface.
6. Apparatus according to claim 5 wherein said coupling liquid
includes a halogenated hydrocarbon.
7. Apparatus according to claim 5 wherein said lens incident and
exit surfaces are coated with thin layers of graphite.
8. Apparatus according to claim 7 wherein the thickness of said
graphite layers is substantially equal to one-quarter wavelength of
ultrasound in said graphite for a fundamental frequency of said
ultrasonic energy generating means.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to ultrasonic generators and more
specifically to those types of generators which include means for
controlling the wavefront curvature of the output beam.
Beamed ultrasonic energy is used for many purposes, such as for
examination of various types of objects for hidden flaws therein.
The usual source of ultrasonic energy is a piezoelectric transducer
driven by an electronic oscillator at an appropriate frequency.
Such transducers can be manufactured from a wide variety of
materials. Quartz transducers are often preferred for high quality
wavefronts. Unaided by other elements, an ultrasonic energy beam of
large cross-sectional area is obtained by a transducer of
substantially the same large area. Transducers, especially those
made of quartz, are very expensive and cost an amount which varies
in proportion to their area. Therefore, it is desirable to use as
small a transducer as possible and somehow expand the small
ultrasonic energy beam emitted therefrom into a beam having a
larger cross-sectional area without degrading the wavefront
quality.
Such expansion has been accomplished in the past by a combination
of two or more ultrasonic lens elements independent of the
transducer, or by a curved transducer and an ultrasonic lens. A
general discussion of ultrasonic lenses may be had by reference to
the book Sonics, authored by Heuter & Bolt, 1955, page 265. Use
of a lens element separate from a transducer is inconvenient to
mechanically support in a manner to prevent misalignment which may
affect beam spread and also affect energy density of the beam.
Therefore, it is a primary object of this invention to provide a
compact unitary transducer-beam control package for generating an
ultrasonic beam having a larger cross-sectional area than the
ultrasonic beam emitted by the transducer.
It is also an object of this invention to provide an ultrasonic
beam lens element with a high refractive power but with reduced
losses from energy reflection.
SUMMARY OF THE INVENTION
These and additional objects are accomplished by an ultrasonic
energy generator according to the present invention which includes
a transducer and a single element solid ultrasonic lens, both
supported by a housing which contains a liquid for coupling
ultrasonic energy therebetween. The transducer is preferably flat
X-cut quartz which emits a collimated ultrasonic beam which strikes
the lens at an incident surface thereof. The radius of curvature of
the incident lens surface is chosen to substantially increase the
divergence of the incident beam. The divergence of the beam passing
through the lens element and the thickness of the lens determine
the cross-sectional dimension of the ultrasonic beam emitted
through an exit surface of the lens. The exit surface has a radius
of curvature chosen to converge to some degree the diverging beam
passing through the ultrasonic lens and thereby produce an output
ultrasonic beam having a predetermined wavefront curvature and
cross-sectional area. The output beam is, therefore, substantially
larger in cross-sectional area than the incident ultrasonic beam
generated by the quartz transducer alone.
Such an ultrasonic energy generator will most commonly be used in a
liquid such as ordinary water or a liquid having a velocity of
sound therein that is close to that of water. Therefore, the lens
element should have the characteristic that sound travels therein
much faster than it does in water so that the radii of curvature of
the lens surfaces may be kept large to avoid generation of unwanted
shear waves. The coupling liquid within the housing may have a
speed of sound lower than that of water so that the radius of
curvature of the incident lens surface may be kept large. With
materials characterized by these relative speeds of sound therein,
the incident lens surface is convex and the exit lens surface is
concave. A preferred combination of materials satisfying these
criteria is aluminum or an alloy with high aluminum content for the
lens element and a halogenated hydrocarbon such as
trichlorotrifluoro-ethane (commercially available as Freon 113) or
carbon tetrachloride for the coupling liquid within the generator's
housing.
According to another aspect of the present invention, it has been
found that the normally high reflection loses at the boundaries of
an aluminum lens may be substantially reduced by applying thereto a
coating of graphite having a thickness at any point along the
surfaces of the lens equal to an odd number of quarter wavelengths
of ultrasonic energy in the graphite. So that an ultrasonic energy
generator according to this invention will have low reflection
losses from the lens element and will be operable at the
fundamental transducer frequency at any of its odd harmonics, a
thickness of graphite that is equal to a quarter wavelength at the
fundamental frequency is applied to both sides thereof.
For a more detailed disclosure of a specific form of the present
invention and for further objects and advantages thereof, reference
should be had to the following description taken in conjunction
with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates in one form a beam expanding ultrasonic
generator according to the present invention;
FIG. 2 is a cross-sectional view of the ultrasonic generator of
FIG. 1 cut in half along a plane passing through section 2--2;
FIG. 3 illustrates a modification of the ultrasonic generator shown
in cross section in FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Use of the term "ultrasonic energy" herein is not to be taken as a
limitation upon the frequency range in which a beam expanding
ultrasonic generator according to the present invention may
operate. The techniques of the present invention apply to
generating and controlling compressional wave energy of any useable
frequency. However, in the more practical applications of
compressional wave energy, such as an object inspection, the higher
sonic frequencies (i.e. those considerably above the audible range)
are more desirable than the lower sonic frequencies. For this
reason, instead of utilizing the more general term "compressional
wave energy," the term "ultrasonic energy" will be utilized in
describing the present invention. This should, however, in no way
limit the scope of the invention.
The specific example of the invention described herein with respect
to the figures is an ultrasonic energy generator utilizing a
piezoelectric transducer which generates a collimated ultrasonic
energy beam of one cross-sectional diameter, which beam is incident
upon an ultrasonic lens which emits a collimated beam of ultrasonic
energy having a much greater diameter. Since a device with these
characteristics has great advantages in ultrasonic work, it is
described in some detail herein but, as may be noted in the
description hereinafter, the scope of applicant's invention is not
limited to the specific embodiment described herein. Referring to
FIG. 1, a beam expanding lens 11 is attached to a transducer
containing housing 13 and generates a collimated ultrasonic beam
through an exit surface 15 of the lens 11.
FIG. 2 shows an enlarged cross-sectional view of the ultrasonic
generator and beam expander of FIG. 1. A flat X-cut quartz
piezoelectric transducer 17 is physically attached to the housing
13 in some appropriate liquid-tight manner. The transducer 17 is
electrically connected to an electronic oscillator (not shown) for
generating a collimated beam 19 having a cross-sectional area
substantially corresponding to the area of the transducer 17. The
collimated beam 19 having a radius h strikes the lens element 11 at
an incident surface 21 thereof. The incident surface 21 is convex
with a radius of curvature r.sub.1 appropriate for the particular
materials used to diverge the rays of the collimated beam 19 into a
diverging ultrasonic energy beam 23. The lens 11 has a thickness d
along its center line. The exit lens surface 15 has a radius of
curvature r.sub.2 appropriate for the particular materials used to
converge the diverging beam 23 into a collimated output beam 25
having a radius h". A useful beam expander for a wide variety of
testing applications is one utilizing a readily available and
economical 2 inch quartz crystal 17 (h = 1 inch) which emits a 6
inch output beam 25 (n" = 3 inches).
The housing 13 contains an ultrasonic coupling liquid 27 to
efficiently transfer energy from the transducer to the lens. Use of
a liquid coupling medium instead of a solid medium provides
flexibility in chosing the shapes of the transducer 17 and of the
incident lens surface 21.
In order to keep the beam expanding ultrasonic generator as compact
as possible, care is chosen in selecting the materials of the
coupling liquid 27 and of the solid lens 11. It is assumed for
purposes of describing herein a specific embodiment of the
invention that the beam expanding ultrasonic generator will be used
in a water medium (n" = 1.0). Aluminum is a preferred material for
the solid lens 11 since it has a low index of refraction relative
to water, n' = 0.235 (defined as the ratio of velocity of sound in
water to the velocity of sound in aluminum). The coupling liquid 27
is chosen from among those which have index of fraction much
greater than that of water, and thus one that is useful in
combination with an alumunum lens. Such a liquid is
trichloro-trifluoroethane (commercially available as Freon 113),
with an index of refraction of n = 2.066. The large difference
between the indexes of refraction of aluminum and Freon, and
between the indexes of refraction of aluminum and water allows the
radius of curvature of the lens surfaces to be kept large as well
as allowing the thickness of the lens to remain small for a given
beam expansion.
Certain well known optical principles may be applied in the design
of a specific ultrasonic beam expander. Applicable optical
principles are discussed in a number of texts, such as Fundamentals
of Optics by Jenkins and White, third edition, especially Chapter 8
thereof. Referring to FIG. 2, a method of design may be illustrated
by tracing an extreme ultrasonic ray 20 from the transducer 17,
through the lens 11 and into the output ultrasonic beam 25.
The ray 20 strikes the incident lens surface 21 at an angle .theta.
with a line normal to the surface. The ray 20 emerges from that
surface at an angle .theta.' with the normal. These angles are
related by Snell's law of refraction, as follows:
sin .theta.' = (n/n') sin .theta. (1)
For a compact beam expanding unit, the angle .theta.' should be
made as large as possible. However, as (sin .theta.') approaches
unity the intensity of undesired shear waves becomes very large and
the transmitted wave decreases in proportion until at unity all
energy reflection is total, which results in poor efficiency of the
lens. Therefore, (sin .theta.') is chosen to be,
sin .theta.' = K.sub.1 (2)
where K.sub.1 is some safety factor less than unity. For instance,
K.sub.1 could be chosen to be 0.75 but its particular value will
depend upon the lens material used and the amount of losses that
can be tolerated.
From the geometry of FIG. 2, it is seen that,
sin .theta. = h/r.sub.1
Combining equations(1), (2) and (3) gives an expression for
determining the radius of curvature r.sub.1 of the incident lens
surface 21:
r.sub.1 = n h/n' K.sub.1 (4)
The next step in designing a beam expander according to the present
invention is to determine the thickness of the lens 11 required to
produce a collimated output beam 25 of the desired diameter. A
graphical method is satisfactory. The angle .theta.' can be
calculated from equation (2). This establishes the path of the
extreme ray 20 through the lens, and also establishes the path of
an extreme ray 22. Ray 22 follows a path which is the mirror image
of the path of ray 20. These two extreme rays are graphically
extended until they are separated by an amount 2h", the desired
output beam diameter. This separation is shown in FIG. 2 between
points 23a and 23b on the extreme rays. Two points of the lens exit
surface 15 are then established to be points 23a and 23b.
The next step is to determine a radius of curvature r.sub.2 of the
lens exit surface 15 which will cause the extreme ray 20 to leave
the lens substantially parallel with the center line, thereby to
produce the collimated ultrasonic beam 25. The ray 20 strikes the
exit surface 15 at an angle .theta." with a line normal to the
surface 15. The ray 20 leaves the exit surface 15 at an angle
.theta..sub.2 with the normal. These angles are related by Snell's
law of refraction, as follows:
sin .theta..sub.2 = (n'/n") sin
From the geometry of FIG. 2, the maximum value of the angle
.theta." is determined by,
.theta." = .theta..sub.2 + .theta.' - .theta. (6)
where .theta.' and .theta. are known from equations (2) and (3).
From the geometry of FIG. 2, it is seen that,
sin .theta..sub.2 = h"/r.sub.2 (7)
The radius of curvature r.sub.2 of the exit lens surface 15 is
determined by equations (5), (6) and (7).
The above described procedure gives the approximate dimensions of a
suitable beam expander. More sophisicated but well-known lens
design techniques can be used to obtain the precise optimum shape
of the curved surfaces to minimize aberrations and distortions of
the wavefront.
The specific embodiment of the invention described with respect to
FIG. 2 expands a collimated ultrasonic beam into another collimated
beam of increased cross-sectional area. One or both of these beams
need not, however, be exactly collimated. In designing a beam
expander where either the beam 19 generated by a transducer or the
output beam 25, or both, are not collimated, the design
considerations discussed herein are equally applicable except that
equations (3) and (7), defining the geometry of ray travel, will be
different. The equations (4) and (8) derived therefrom will also
take a different form.
It may also be noted that the transducer-lens assembly illustrated
herein is insensitive to wavelength. That is, the beam emitted by
the transducer will be expanded the same amount regardless of the
ultrasonic frequency emitted by the transducer.
The use of materials as described herein with widely varying
indexes of refraction provide for a compact beam expanding unit.
There is a problem, however, in such a combination since the
impedances of the materials used differ a great deal which thus
results in an impedance mismatch at the incident lens surface 21
and at the exit lens surface 15. As used in the art, acoustic
impedance of a material is defined as the product of its density
and the speed of sound therein. A mismatch of impedances of two
materials creates large energy reflections at an interface between
the two materials, as is well known. In the configuration described
with respect to FIG. 2, reflective losses at the two interfaces
total about 78 percent of the ultrasonic energy striking the
incident lens surface 21. Therefore, a large amount of energy is
required to be generated by the transducer 17 in order to obtain an
output beam 25 with sufficient energy for most applications. To
significantly reduce these losses, each surface of the lens 11 is
coated with a thin layer of material having an impedance which is
nearly a geometric mean between the impedances of the two materials
joined. It has been found that graphite has an impedance which is
substantially the geometric mean between the impedances of the
aluminum lens and the fluids that contact the lens on either side
thereof. High density graphite is preferred, having a density in
the neighborhood of 1.8 grams per cubic centimeter. The lower
density graphite is also satisfactory but is not preferred because
of its higher absorption losses. As shown in FIG. 3, the incident
lens surface 21 may be coated with a thin layer of high density
graphite 31 to substantially reduce reflection losses at the
Freon-aluminum interface without significantly affecting the
refractive characteristics thereof. Similarly, the exit lens
surface 15 may be coated with a thin layer of high density graphite
33 to substantially reduce the reflections of ultrasound at the
aluminum-water interface. The reflective losses of such a lens are
in the order of 1 percent.
The graphite layers 31 and 33 should have a thickness which is
substantially an odd number of one-fourth wavelengths within
graphite in order to prevent rather substantial losses therein.
Also, since the absolute thickness of the graphite layers should be
as small as possible to reduce absorption, it has been found
preferable that the layers have a one-fourth wavelength thickness.
It should be noted that the thickness of these layers may vary
depending on position along a surface of the lens since the
thickness must be measured along a sound ray passing therethrough
which does not necessarily make the same angle with the surfaces of
the layer at all positions. Equations for determining graphite
thickness as a function of position along the lens surface are
given in the book Waves in Layered Media, L. M. Brekhovskikh,
Academic Press (1960). Equation 5.26 at page 50 is especially
useful here.
A one-fourth wavelength thickness has a further advantage in
situations where the transducer may be operated at either its
fundamental or an odd harmonic since a layer having a one-fourth
wavelength thickness at the transducer fundamental frequency will
have a thickness equal to an odd number of one-fourth wavelengths
at any of the transducer's odd harmonics. The typical fundamental
frequency for object flaw examination is one megacycle, with the
possibility of the same transducer emitting odd harmonics thereof
upon adjusting the driving oscillator to 3, 5, or 7, etc.
megacycles.
While there has been described a preferred embodiment of the
invention, it is understood that further modifications are possible
without departing from the spirit and scope of the invention as set
forth in the appended claims.
* * * * *